Known in the art is an internal combustion engine performing lean combustion such as a diesel engine. It has been proposed to arrange in series an NOx storing/reducing catalyst device for purifying the NOx in the exhaust passage of this internal combustion engine and a particulate filter for sufficiently purifying particulate at the downstream side of the NOx storing/reducing catalyst device (for example, refer to Japanese Unexamined Patent Publication No. 2005-90256, Japanese Unexamined Patent Publication No. 2005-133610, Japanese Unexamined Patent Publication No. 7-243322, Japanese Unexamined Patent Publication No. 11-200852, Japanese Unexamined Patent Publication No. 10-252455, and Japanese Unexamined Patent Publication No. 6-117221).
If making the particulate filter carry a NOx storage catalyst, NOx can be stored and purified by reduction. Additionally, the NOx storage catalyst can also function as an active oxygen release agent. The active oxygen released from the NOx storage catalyst can be used to purify the trapped particulate by oxidation at a low temperature. However, if a large amount of particulate is trapped at one time in the particulate filter such as during engine acceleration operation, there are times when just the active oxygen released from the NOx storage catalyst cannot sufficiently purify the trapped particulate by oxidation. Therefore, the amount of particulate deposition gradually increases. If a set amount or more of particulate deposits at the particulate filter, the exhaust resistance of the particulate filter will have a detrimental effect on the engine output, therefore, when the set amount of particulate deposits at the particulate filter, it becomes necessary to forcefully burn off the deposited particulate. To do so, the particulate filter must be raised in temperature to about 600 degrees C.
Such temperature elevation control of the particulate filter generally is performed by injecting additional fuel into the cylinder at the expansion stroke or exhaust stroke or feeding additional fuel to the engine exhaust system at the upstream side of the NOx storing/reducing catalyst device. This additional fuel of the temperature elevation control, when passing through the NOx storing/reducing catalyst device, is made to burn using oxygen in the exhaust gas by the oxidation catalyst carried at the NOx catalyst reduction catalyst device. The combustion heat raises the exhaust gas in temperature to 600 degrees C. or more. The exhaust gas is then made to flow into the particulate filter and makes the temperature of the particulate filter rise.
Therefore, for each temperature elevation control of the particulate filter, the NOx storing/reducing catalyst device is also simultaneously raised to 600 degrees C. or more, whereby the NOx storing/reducing catalyst device ends up deteriorating due to the heat.